PROJECT SUMMARY The receptor tyrosine kinases ErbB4 and ErbB3 can protect intestinal enterocytes from apoptosis, but their potential roles in inducing stem cell regeneration after an insult?arguably of equal importance in the face of challenge?are not known. Here, we aim to define the function of ErbB3 and ErbB4 in post-injury recovery of intestinal stem cells, and thus in regeneration of the epithelium. ErbB4 expression is high in regenerating enteroids, and the ErbB4 ligand NRG4 promotes crypt plating efficiency. ErbB4-null enteroid cultures have a compromised intestinal stem cell niche with loss of the rapidly-cycling stem cell marker Lgr5 and reduced Paneth cell numbers. Similar to ErbB4, ErbB3 promotes enterocyte survival, but unlike ErbB4 it suppresses secretory cell differentiation; for example, ErbB3-null enteroids have an expanded Paneth cell census. Thus, the two neuregulin receptors seem to play complementary but distinct roles in regulating renewal and development in the crypt. Preliminary data suggest that both of these receptors are induced during recovery from injury models in vitro and in vivo, and loss of either perturbs recovery. We will build on our published and preliminary data to test the hypothesis that signaling through ErbB4 and ErbB3 promotes balanced regeneration of the intestinal epithelium after injury. We will (1) define the impact of ErbB4 or ErbB3 loss or activation on intestinal epithelial regeneration after injury, (2) determine the molecular mechanisms linking ErbB4 and ErbB3 to intestinal stem cell regeneration, especially connections to the fundamental regulators of stem cell self-renewal, Wnt and Notch. These studies will advance basic understanding of how intestinal stem cells and the stem cell niche are repaired, as well as identify novel regulatory mechanisms that could be future therapeutic targets to drive intestinal regeneration after injury.